This invention relates to a trimaran construction and, in particular, to a construction which is adapted to be used in large sized vessels but which is not restricted to these. Some large sized vessels to which the invention is applicable are those which are used for vehicular and other freight and passenger ferries. In this specification, we shall refer to ferries but it is to be understood that this is purely exemplary.
Historically, ferries were mono-hull vessels and whilst these were satisfactory as far as stability and power usage was concerned, they suffered from the disadvantage of having a restricted beam, and thus carrying capacity.
More recently many ferries have been of a catamaran construction which provide good transverse stability, enhanced carrying capacity because of the spacing between the two hulls with a large portion of the load being carried between the hulls and limited drag because the two hulls can be made relatively narrow.
However, whilst having good transverse stability, catamarans are very xe2x80x98stiffxe2x80x99 which can be uncomfortable in beam seas. Also the ride control, which is often fitted to such vehicles has to produce large forces to reduce the motions.
The object of the present invention is to provide a trimaran construction which has benefits over both mono-hulls and catamarans.
The invention in its broadest sense comprises a trimaran vessel having a central hull and two outer hulls each small and of less draft compared to the central hull, the arrangement of the vessel being such that the draft of the vessel lessens when the vessel is moving at speed due to dynamic lift developed by the movement and whereby because of the change in draft the outer hulls have little if any effect on buoyancy of the vessel when moving at speed.
In this specification, the term xe2x80x98moving at speedxe2x80x99 indicates that the vessel is being driven at a speed sufficient to develop the lift which is required to satisfy the characteristics required by the invention.
It is preferred that the vessel has at least one lifting surface which provides the necessary lift to lessen the draft when the vessel is moving at speed.
The arrangement may be that when the vessel is moving at speed, the hydrodynamic lift provided by the lifting surface(s) is equivalent to the loss of buoyancy provided by the central and the outer hulls.
It may be preferred that there is provided at least one foil or the like to provide the lifting surface.
Alternatively, the lifting surface may be provided by making the central hull, at least, a planing hull.
In practice, the outer hulls are small relative to the central hull and provide, at rest only a small portion of the total buoyancy of the vessel.
In a preferred aspect of the invention the side hulls have stepped profiles which assist in further reducing drag.
In order that the invention may be more readily understood we shall describe, in relation to the accompanying drawings, a particular form of trimaran made in accordance with the invention.